Demystifying fertility

In the Palaeolithic period, the earliest human representations already symbolised fertility. The topic continues to occupy our minds 30,000 years later. “Fertility is both very mysterious and very important for all people who want to become parents,” says Anna Surbone, head of medically assisted reproduction at CHUV. “For a long time, how human reproduction works remained hidden. And since it was a very personal matter, we didn’t dare to talk about it.” This lack of information has led to the spread of many beliefs. Methods, more or less outlandish, to give birth to boys have, for example, been peddled in our societies, where concern about having a male heir was predominant. 

“Contrary to what patients imagine, they do not need to keep their legs raised after intercourse,” says Anna Surbone. “Sperm move quickly in the female reproductive system; gravity does not change their direction.” Sperm can be found in the cervical canal within seconds of ejaculation, regardless of sexual position, as reported by the American Society for Reproductive Medicine in 2022. No sexual practice affects the sex of the offspring either.

The image of the victorious sperm, the fastest and 'strongest' in piercing the maternal cell's membrane, is widespread. We now know, however, that the egg does not play only a passive role. It chooses, to some extent, the sperm. On the one hand, according to 2020 European research, it sends chemical signals to attract compatible male cells. Lastly, proteins on its surface, some of which remain poorly understood, play a role in the sperm's ability to fuse with it and then form the fertilised egg.

“Falling fertility is a reality from the age of 35.” Before age 30, a woman has an 85% chance of conceiving within a year, according to the Upsala Journal of Medical Sciences. This rate drops to 75% at age 30, 66% at age 35, and 44% at age 40. At the same time, the rate of unplanned pregnancies among women in their forties exceeds that among those in their thirties: 31% compared with 27.5%, according to US statistics. “Because it is difficult to conceive a child after age 40 without medical help, some women may think that they can do without contraception.” Symptoms of premenopause, such as irregular cycles, may also mislead some women.

Biological sex has long been seen as a kind of lottery, with equal chances of having a girl or a boy. This conception was questioned last summer in the journal Science Advances. The research team found a correlation between giving birth to a series of same-sex children and older maternal age. After age 29, women are more likely to have only girls or only boys.

To go even further

«Knowledge about the impact of age on fertility: a brief review», Ilse Delbaere, Sarah Verbiest, Tanja Tydén (2020)

«U.S. Pregnancy Rates Drop During Last Decade», CDC, National center for Health Statistics (2023)

«Is sex at birth a biological coin toss? Insights from a longitudinal and GWAS analysis», Siwen Wang, Bernard A. Rosner, Hongyan Huang, Janet W. Rich-Edwards, Francine Laden, Jaime E. Hart, Kathryn L. Penney, Jorge E. Chavarro, (2025)

«Chemical signals from eggs facilitate cryptic female choice in humans», John L. Fitzpatrick, Charlotte Willis, Alessandro Devigili, Amy Young, Michael Carroll, Helen R. Hunter and Daniel R. Brison (2020)

«Optimizing natural fertility: acommittee opinion», American Society for Reproductive Medicine, Birmingham, Alabama (2017)